An important class of semiconductor laser device is the buried heterojunction type, and an important subcategory of that type includes lasers which employ a superlattice as the active layer. Such devices typically have a central stripe in the active layer of reasonably precise dimension which serves as a resonator, and a pair of regions which bound the central stripe of higher band gap and lower refractive index. One reason for the interest in superlattice buried heterojunction lasers is that the central stripe can be formed fairly simply, by controllably diffusing impurities into the active layer to disorder the regions bounding the central stripe. The diffused impurities, by a process sometimes known as diffusion induced disordering, tend to disorder the crystal structure of the superlattice, achieving the desired band gap and refractive index discontinuity between the disordered and non-disordered regions.
A known process for controllably disordering a superlattice is illustrated in FIGS. 3(a)-3(d). FIG. 3(a) shows a multiple layer structure formed during an epitaxial growth process including a GaAs buffer layer 2 epitaxially grown on a GaAs substrate 1. Thereafter, individual layers of GaAs and Al.sub.x Ga.sub.1-x As of about 100 .ANG. thickness are alternately grown on the GaAs buffer layer 2 to produce an AlGaAs series superlattice 3 of about 0.5 .mu.m or less in thickness. As is well known, in the superlattice the GaAs layers serve as quantum wells and the AlGaAs layers as quantum barriers. In a typical application, the aluminum and gallium molar proportions in the AlGaAs can be about equal, i.e., x=0.5.
After the conclusion of the epitaxial growth process, a further layer is deposited to serve as a source of impurities for the disordering operation. In an exemplary application, the wafer is placed into a vapor plating or sputtering apparatus and as shown in FIG. 3(b), a silicon film 4 is produced by a vapor plating or sputtering. The silicon film is, in some cases, covered with a cap layer such as SiO.sub.2. Selective control of the diffusion is accomplished by removing a central stripe from the silicon film 4. That is typically accomplished by depositing a mask using photolithographic techniques, then etching away the central stripe to leave a pair of regions of the film 4 as illustrated in FIG. 3(c).
Having thus deposited and patterned the source of silicon impurities, various techniques are available for diffusing the impurities from the layer 4 into the superlattice. For example, either a closed tube method or an open tube method can be employed. When using the closed tube method, the wafer, in the condition illustrated in FIG. 3(c), is placed into a quartz tube of 16 mm diameter and 20 cm length together with arsenic of about 40 mg. The quartz tube is exhausted to vacuum, and heated up to about 850.degree. C. Under these conditions, the vapor pressure of As.sub.4 is about 0.3 atm. Silicon from the silicon film 4 then diffuses into the wafer to a depth of about 1 .mu.m when subjected to a temperature of 850.degree. C. for about 1 hour. Longer annealing times or higher annealing temperatures cause the impurities to diffuse further into the superlattice. Diffusing of the silicon impurities into the crystal structure of the superlattice serves to disorder the superlattice. More particularly, the impurities cause the aluminum and gallium molecules in the lattice to move, and the alternating layers of GaAs and Al.sub.0.5 Ga.sub.0.5 As are effectively mixed, producing an Al.sub.0.25 Ga.sub.0.75 As crystal structure of uniform composition. FIG. 3(d) depicts the wafer after the disordering, in which the silicon diffused portion 5 has produced a disordered superlattice consisting of a Al.sub.0.25 Ga.sub.0.75 As doped with silicon impurities bounding a central undoped stripe which retains its ordered superlattice character.
When using an open tube method, the wafer is heated while flowing hydrogen diluted arsine (AsH.sub.3) or argon diluted arsine of a 10% concentration at a flow rate of about 300 cc/min. By this open tube method, the silicon is diffused similarly as in the closed tube method, and the superlattice is controllably disordered in regions 5 as shown in FIG. 3(d). Additionally, this open tube disordering method may also be conducted using germanium rather than silicon as the source of diffusion impurities. The process differs primarily in that film 4, which is deposited after the epitaxial growth phase, is germanium rather than silicon.
As noted above, the reason that controllably disordering a superlattice is important is that the disordered region has a higher energy band gap than the non-disordered superlattice and also has a lower refractive index. Those features are very useful in a laser device because the higher band gap keeps carriers confined within the non-disordered superlattice, and the lower refractive index aids in confining photons within the non-disordered region in waveguide-like fashion. FIG. 4 shows a method of producing a semiconductor laser device using the above-described prior art disordering technique. As shown in FIG. 4(a), an AlGaAs lower cladding layer 9a, an AlGaAs series superlattice active layer 10, an AlGaAs upper cladding layer 9b, and a GaAs contact layer 11 are epitaxially grown on the GaAs substrate 1. Following the epitaxial growth phase, the wafer is placed into an apparatus for vapor plating or sputtering, and as shown in FIG. 4(b) a silicon (or germanium) film 4 is produced on the upper cladding layer. The film is patterned photolithographically, then etched as shown in FIG. 4(c), to remove a strip from the film 4. The wafer is then annealed by a closed tube method or an open tube method, as previously described, resulting in the diffusion of silicon or germanium impurities to disorder the superlattice as shown at 5 in FIG. 4(d). Next, as shown in FIG. 4(e), the film 4 is removed by etching, and as shown in FIG. 4(f), electrodes 14 and 15 are then attached in electrical contact with the semiconductor to complete the semiconductor laser device. It it important that the silicon-containing film be carefully removed before the electrodes are applied because they prevent the formation of a good ohmic contact between the electrode and the semiconductor crystal structure.
In addition to the solid phase diffusion techniques described above, vapor phase diffusion can also be used with certain materials, such as zinc or sulphur, to disorder the superlattice. Since zinc or sulphur have a higher vapor pressure than silicon or germanium, zinc or sulphur may be diffused into the wafer from the vapor phase by heating a quartz tube containing zinc or sulphur to cause diffusion and the resulting disordering of the superlattice. However, if the vapor phase diffusion process is to controllably disorder the superlattice, it is necessary to produce a diffusion mask on the semiconductor prior to diffusion, then to remove the mask before the electrodes can be applied to the laser.
Typical diffusion masks are SiO.sub.2 or Si.sub.3 N.sub.4, and as can be appreciated thin films of those materials are typically deposited by chemical vapor deposition, a technique typically as complex as the sputtering or evaporation used in connection with the silicon or germanium films. After the film is applied, it must then be photolithographically patterned and etched to remove the regions bordering the central stripe. Then following the vapor phase diffusion to the appropriate depth, the SiO.sub.2 or Si.sub.3 N.sub.4 masking material must be removed by a process such as etching. It will therefore be appreciated that although the dopant is carried by a gas in vapor phase diffusion, rather than by a deposited film as in solid phase diffusion, the requirement for depositing and forming the diffusion mask makes the two processes of about equal complexity. Furthermore, the results of vapor phase diffusion tend to be less than satisfactory since the steepness of the boundary surface between the disordered portion of the superlattice and the unaffected portion of the superlattice is lower relative to the steeper boundary surface results realized from solid phase diffusion. As a result, the discontinuity in the refractive index and in the band gap is not as sharp as is desired and thus the disordered region is less efficient in confining electrons and photons in the non-disordered region.